1. Field of the Invention
The present invention relates to a semiconductor photosensor device for an illuminance sensor, for example, a semiconductor photosensor device which can detect an illuminance falling in a wide range of several lux to several tens of thousand lux.
2. Related Art
A semiconductor photosensor device (illuminance sensor) is a photosensor which outputs a linear output depending on an ambient illuminance (brightness). Mainly in a mobile phone, the illuminance sensor is mainly used for ON/OFF control of LEDs (Light Emitting Diodes) of a back light or an operation unit (key unit) of a liquid crystal screen depending on an ambient illuminance (brightness). For example, the illuminance sensor turns off some light emitting diodes of the back light or the key unit when the ambience is brought and turns on the light emitting diodes or performs brightness control or the like to suppress unnecessary power consumption when the ambience is dark.
An illuminance sensor of a conventional mobile phone has been used for ON/OFF control of light emitting diodes of a key unit. Therefore, such illuminance sensor is designed to output a linear output within a low-illuminance range of several lux to hundred lux.
At the present, not only ON/OFF control of light emitting diodes of a key unit, but also control of the back light of a liquid crystal screen are demanded. This is because a power consumption increases due to the advance of full-color liquid crystal of mobile phones. Since a transmissive liquid crystal screen uses a back light on the back side of the screen as a light source, a chroma saturation is high. For this reason, the transmissive liquid crystal screen can be easily viewed in a dark room, but the screen is dark in a bright room. In order to cope with this drawback, the back light is controlled depending on ambient brightness. A high illuminance up to several tens of thousand lux must be detected to control the back light.
However, since a high-illuminance photodiode which can detect an illuminance up to several tens of thousand lux has low sensitivity, the photodiode cannot be detect low-illuminance light. On the other hand, since a low-illuminance photodiode which can detect an illuminance up to 100 lux has high sensitivity, the photodiode is saturated by high-illuminance light. For example, an illuminance sensor used for control of some light emitting diodes of the key unit of a mobile phone cannot control the back light of a liquid crystal screen. Furthermore, an illuminance sensor used for control of the back light of a liquid crystal screen cannot control light emitting diodes of a key unit.